I'm going to start a little index of useful solutions for Dynamics 365 - Add a comment with your recommendations and i will check them out and get them added

Do you have multiple files to upload to a record?

Drag and Drop Code from CRMXpress


Autonumbering on more than the standard entities? Complex rules?

eBECS AutoNumber for Dynamics 365


Multiple people working in 1 instance? Track the changes

Change Tracking Solution from Microsoft Labs


Do you have integrations setup Via Scribe

Integration Manager from Scribe


Need to run scheduled workflows? fire in some FetchXML and away you go

Workflow Scheduler from Lucas Alexander